Natural circulation of cathode metal of electrolytic cell

ABSTRACT

A natural circulation process for the removal of the product of electrolysis from a flowing molten metal cathode wherein such removal of the product of electrolysis is achieved through the flash vaporization of the metal of the electrolyte salt in a vacuum still with recirculation of the cathode metal to the electrolytic cell by gravity due to increased density associated with the vaporization of the metal of the electrolyte salt and the cooling of the cathode metal from the flash vaporization. The process is particularly applicable to the separation of sodium from a lead-sodium system, the sodium being separated containing approximately 0.1-1 percent lead impurity.

United States Patent 72] Inventors George G. Day

Maderia Beach, Fla.; Charles F. Bonilla, Tenafly, NJ. [21] Appl. No.808,404 [22] Filed Mar. 19, 1969 [45] Patented Nov. 16, 1971 [73]Assignee F. Barry Haskett New York, N.Y.

[54] NATURAL CIRCULATION 0F CATIIODE METAL 0F ELECTROLYTIC CELL 7Claims, 2 Drawing Figs.

[52] US. Cl 204/68, 204/220, 204/237, 204/245, 204/250 [51] Int. Cl C22d3/06, C22d 3/02 [50] Field of Search 204/68, 219, 220, 250, 66, 245, 237

[56] References Cited UNITED STATES PATENTS 1,597,231 8/1926 Haynes204/247 X 2,862,863 12/1958 Griffith 204/68 X 3,167,492 1/1965Szechtrnan 204/68 3,265,606 8/1966 Marullo et al. 204/220 X 3,502,5533/1970 Gruber 204/245 X Primary Examiner-John H. Mack AssistantExaminer-D. R. Valentine Attorney-Sherman and Shalloway ABSTRACT: Anatural circulation process for the removal of the product ofelectrolysis from a flowing molten metal cathode wherein such removal ofthe product of electrolysis is achieved through the flash vaporizationof the metal of the electrolyte salt in a vacuum still withrecirculation of the cathode metal to the electrolytic cell by gravitydue to increased density associated with the vaporization of the metalof the electrolyte salt and the cooling of the cathode metal from theflash vaporization. The process is particularly applicable to theseparation of sodium from a lead-sodium system, the sodium beingseparated containing approximately 0.1-1 percent lead impurity.

PATENTEDunv 15 Ian 3, 620. 942

a! H FIG. 2

q T M I 1 INVENTORS W CHARLES F. BONILLA 3a 7 GEORGE 6. DAY

*"' BY K 6m gala/W7 ATTORNEYS NATURAL CIRCULATION OF CATHODE METAL OFELECTROLYTIC CELL The present invention is directed to a naturalcirculation system for the recovery of the metal of the electrolyte saltemployed in the electrolysis process and for the recirculation of theflowing metal cathode employed therein; more particularly, the processof the present invention comprises a natural circulation system for theseparation of a substantially leadfree sodium vapor from a recirculatinglead-sodium cathode metal, i.e. lead containing about l percent sodiumgenerated in the electrolytic cell. This process also serves to controlthe composition and temperature of the cathode metal, and also to someextent the purity of the recovered sodium, or other metal of theelectrolyte salt.

In electrolytic processes employing a molten metal cathode, e.g. a heavymetal such as tin or lead and a fused electrolyte salt, such as sodiumor potassium chloride, a mixture is formed by the dissolving of theelectrolyte metal in the molten cathode, generally forming a liquidalloy containing from about 5-20 percent by weight of the electrolytemetal, with the remainder being the cathode metal. Thus, for example, inthe case employing a molten lead cathode in the electrolysis of sodiumchloride, during the electrolysis process sodium metal deposited at thecathode, which is the flowing molten lead,

becomes dissolved in the lead.

Accordingly, in order that the flowing molten lead cathode can berecirculated and reused in the electrolysis process, it is necessary toremove the product of the electrolysis, that is the sodium metal, fromthe lead as fast as it is produced.

Sodium-lead mixtures predominating in lead have high boiling points.Thus, for example, a mixture containing percent sodium and 90 percentlead by weight has a boiling point of about 970 C., whereas pure sodiumboils at 883 C. at atmospheric pressure. Accordingly, the high operatingtemperatures necessary for separating sodium and lead favor the use ofcompact apparatus in order to minimize heat losses. Similarly, it hasbeen found desirable to use special materials for construction, forexample, stainless steel, refractories or even refractory alloys orceramics, to resist internal corrosion by sodium-lead alloy and externaloxidation by air at the high operating temperatures required.

Two primary processes have been developed for the circulation of thelead cathode alloy from the electrolytic cell to the sodium vaporizerand back to the cell. One of these involves the use of a mechanicalpump, usually of the type known as a centrifugal pump. A number ofprecautions, however, should be taken in selecting a pump of this type.For example the bearings should generally be above and outside of themain casing, with inert gas under pressure keeping the liquid metalbelow the bearings. In addition, the shaft must be long, and generallyis tapered, to minimize weight yet achieve stiffness to avoid vibration,and the pump should presumably be run at a low speed, to avoidcavitation. Furthermore, the materials of construction must be carefullyselected for the various components of the pump and system. All of thesefactors and criteria combine to make such a system for the circulationof the cathode metal, e.g. lead, a fairly complicated and expensive one.

A second method which has been proposed for handling the liquid metalsinvolved in the electrolysis process involves the use of anelectromagnetic pump of an alternating current or direct current type.This type of pump appears to work quite satisfactorily providing thetemperature is not too high and the electrical conductivity of theliquid metal is good. However, this is not the case in the distillationof sodium and similar electrolyte metals from an alloy of suchelectrolyte metal and a molten cathode metal such as lead, since itselectrical conductivity is low and thus the performance and efiiciencyof the pump are poor. In addition, electrical connections, and usuallywindings, are necessary in this type of pump which generally involvesspecial insulation to stand the temperatures, with air-cooling andpossibly water-cooling required for some of the parts. Again, thisresults in a complicated and expensive system.

Even though these two methods of pumping the cathode metal alloy arefairly well known, they both require pumping equipment, driving power,and substantial piping external to the electrolytic cell, and thus aremoderately expensive, and require considerable maintenance, and have aninherent possibility of failure. Accordingly, a simpler, and thus morefoolproof and economical method of transporting the cathode metal alloyinto a vaporization chamber and back to the cell is desirable. Such asimple, yet efficient process for the recirculation of a flowing cathodemetal alloy has been provided with the process of the present invention.

The basic feature of the present invention is that the objective of therecirculation of the cathode metal alloy, which is to accomplishvaporization and separate recovery of the sodium produced byelectrolysis, is employed to cause the recirculation. This isaccomplished by the fact that when the cathode metal alloy enters thevaporization chamber, in which the absolute pressure is maintained belowthe vapor pressure of the sodium in the cathode metal alloy by means ofa vacuum, some of the sodium flashes" out of the cathode metal alloyuntil the cooling down of the alloy and its loss of sodium have causedits vapor pressure to fall to the pressure being maintained in thevaporization chamber by a sodium vapor conidenser and a subsequent inertgas vacuum pump. In this vaporization of sodium and cooling of thecathode metal alloy its density rises appreciably for both reasons.Therefore, the molten alloy leaving the vaporization chamber will have asubstantially higher density than the molten alloy entering it. Bymaking the piping through which the molten alloy flows large enough, therate of recirculation can be made as large as needed. Furthermore, thispiping can be shorter than when pumps are used. This method ofcirculation is evidently simple and economical, and represents aconsiderable advance over conventional pumping technology for theunusual and high temperature liquid metals herein involved.

This method of recirculation of the cathode metal alloy could beemployed in conjunction with any method of recovery of the sodium. Forinstance, a recent process for separating sodium and similar electrolytemetals from cathode metal alloys, improving the disadvantageousprocesses of the prior art discussed above, can be found in a copendingapplication Ser. No. 586,859 filed Oct. 14, 1966, now US. Pat. No.3,484,233, in the name of Charles F. Bonilla. Such a process is onewhich allows for the recovery of pure sodium metal, that is, sodiummetal having less than one part per million lead. The process andapparatus disclosed in such application involves a two-stage ordouble-efiect" process by which through two (or more) successiveevaporations with intermediate total condensation it is possible toreduce the lead content of a sodium vapor to a point of high purity.

However, it is another objective of the present invention to combine thesimple gravity circulation system for the cathode metal alloy of asingle cell with a simple sodium recovery system so as to produce sodiumacceptable for some uses in the very simplest and most economicalmanner. Thus, the requirements of the vaporization phase of the processwould only be, according to this consideration, to obtain the necessarydensity change in the cathode metal alloy to maintain the recirculation,while allowing the sodium produced to contain moderate amounts ofimpurities, and in particular lead. This is achieved most simply byproviding only a single vaporization stage or effect." Furtherpurification of the sodium could then be obtained by a latervaporization operation, probably in a single piece of equipment for thewhole plant.

The proper elevation of the level of the inlet pipe in the vaporizationchamber, or of the liquid level of the main pool,

(whichever is higher) over the level in the electrolytic cell is thedifference between those two desired pressures divided by the density ofthe desired composition of cathode metal alloy. Thus, the cell may be atatmospheric pressure (760 torr) and the vaporization chamber vapor maybe at torr absolute pressure (corresponding to approximately l0 percentsodium in lead at 810 C.). The difference in pressure is 690 torr, to becounteracted by the lead of the 10 percent sodium alloy plus negligiblefriction. The density of this alloy at approximately 10.2 percent sodiumand 830 C. is 5.5 grams/cc. Therefore, to maintain these conditions thelevel in the vaporization chamber should be approximately 69 l3.6/5.5=I70 centimeters (5.6 feet) above that in the electrolytic cell, orseveral centimeters less to allow for some friction in the pipe.

Thus, in accordance with the present invention, a process has been foundwhich simply, efiiciently and economically causes the circulation ofcathode metalelectrolyte metal alloy, through a single vaporizationchamber without a pump, to produce an electrolyte metal product byvaporization containing from about 0.1 to about 1 percent by weight ofthe cathode metal impurity, with the cathode metal being returned to thedesired composition for recirculation to the electrolytic cell. Such aprocess developed in accordance with the present invention takesadvantage of the increase of density of the cathode metal associatedwith the flash evaporation of the electrolyte metal caused to occur inthe vaporization chamber and the cooling therewith of the alloy system,to achieve the natural circulation of the present invention.

Accordingly, it is a principal object of the present invention toprovide a process for the natural circulation of the cathode metalthrough an electrolytic cell, including separation of electrolyte metaltherefrom in a manner which eliminates the normally required pump andother inherent deficiencies and disadvantages of prior known processes.

It is a further object of the present invention to provide such aprocess for the natural circulation of the cathode metal in anelectrolytic cell which in a simple, efficient and economical mannerallows for the production of an electrolyte metal containing from about0.1 to about 1 percent by weight of the cathode metal, which in the caseof the production of sodium produces a material which can be effectivelyemployed, for example, in the preparation of tetraethyl lead and similarproducts.

It is yet a further object of the present invention to provide a processfor the circulation of molten alloys from an electrolytic cell to avaporizer and back to the cell, the recirculation of the cathode metalbeing effected by the increase in density due to the vaporization of thelighter electrolyte metal 'and cooling of the cathode metal composition.

Still further objects and advantages of the process of the presentinvention will become more apparent from the following more detaileddescription thereof.

In accordance with the process of the present invention it has beendiscovered that it is possible to efficiently, economically and simplyprovide a recirculating system in an electrolytic process for thepurification and recirculation of the molten metal cathode. As indicatedpreviously, the process of the present invention is particularlyapplicable when employing molten lead as the flowing cathode metal. Moreparticularly, the process of the present invention is exceptionallysuitable for the electrolysis of a fused or molten salt, e.g. sodiurnchloride in a horizontal electrolytic cell wherein the flowing leadcathode flows along the bottom with the electrolyte salt floating on topof the same. Such a typical horizontal electrolytic cell, for examplecan be found in U.S. Pat. No. 3,l04,2 l 3. In this regard, the processof the present invention directed to the natural recirculation of thecathode metal is applicable to all and any of those electrolytic cellswhich conventionally employ a molten metal cathode.

In accordance with the process of the present invention, the drivingforce involved in the separation of the electrolyte metal, sodium, fromthe alloy of cathode metal, lead, and sodium is in actuality a doubleone. Thus, for instance, the natural recirculation of the alloy havingthe decreased sodium content is effected through gravity by the increasein density of the sodium-lead alloy when the lighter metal sodiumvaporizes out of it. A further increase in the density of thesodium-lead alloy assisting the same to be effectively recirculated tothe electrolytic cell, occurs due to the cooling of the alloy, from theflash vaporization of the sodium, which extracts sensible heat from thealloy. It is this combined driving force increasing the density of thesodium-lead alloy for recirculation to the electrolytic cell that allowsthe same to be easily and naturally circulated through gravitationalforces only. Thus, the necessity for a pump is completely eliminated inaccordance with the process of the present invention.

Thus, in accordance with the process of the present invention, thecirculation of a cathode metal is caused to take place in a closed loopthrough changes in the density of the liquid alloy resulting fromchanges in composition and changes in temperature. Such a process iscarried out under vacuum, (i.e. low pressure) conditions, the vacuumconditions permitting that the cathode metal-electrolyte metal alloy canbe pushed up from the discharge point of the electrolytic cell into avaporizer of a flash still provided for effecting the flash vaporizationof the electrolyte metal from the alloy of electrolyte metal and cathodemetal by the atmospheric pressure of the chlorine in the electrolyticcell. In the vaporizer operated under vacuum conditions, a portion ofthe electrolyte metal, usually sodium is flashed off, the vapors thenpassing through a condenser where the electrolyte metal vapor iscondensed to form the liquid electrolyte metal. In such a processwherein a single flash vaporization or flash distillation of theelectrolyte metal and cathode metal is employed, the condensedelectrolyte metal will generally contain from about 0.1 percent to aboutl percent by weight lead as a directly produced product of the processof the present invention which can be effectively employed, for examplein the production of tetraethyl lead.

The heat required for the vaporization of the electrolyte metal issupplied by the sensible heat of the main body of the alloy ofelectrolyte metal and cathode metal drawn into the vacuum still orevaporator. Accordingly, upon the evaporation of the electrolyte metalfrom the main body of alloy, the temperature of the main body of alloywill decrease. This drop in temperature may be up to 20 C., or moredepending upon the temperature of the alloy and the electrolyte metalcontent thereof.

Similarly, in view of the vaporization of the electrolyte metalcontaining only a very minor amount of the cathode metal, thecomposition of the main body of alloy will change due to the flashvaporization. This change, of course, will bring about an alloycomposition which will contain a lower percentage of the lighterelectrolyte metal, sodium, and a higher percentage of the heavier metal,lead. Accordingly, for this reason, also, the density of the main bodyof alloy of electrolyte metal and cathode metal will change due to theflash vaporization of the electrolyte metal from the main body of alloy.This combined driving force of increase in density due to both alowering of the temperature of the main body of alloy and a change inthe composition of such main body, will result in a return by gravity ofthe alloy to the electrolytic cell. Thus, the more dense alloy resultingfrom the vaporization of the electrolyte metal in the flash evaporatorwill flow back to the electrolytic cell at an entrance point thereof dueto the increase in density of the alloy composition.

The change in the composition of the alloy of cathode metal andelectrolyte metal need not be great for operation of the process of thepresent invention. Thus, for example, on an industrial scale, theelectrolytic cell is generally operated with only a small increase ofthe electrolyte metal content of the alloy and accordingly, only a smallamount of such metal need be evaporated prior to recirculation of thecathode metal to the electrolytic cell. In this respect, for example, anindustrially operated electrolytic cell c'an preferably be operated witha cathode having a content of approximately percent lead and 10 percentsodium. The increase in the sodium content of the cathode metal uponexiting from the electrolytic cell may be only a matter of 2/10 of 1percent or less and it is only this amount which must be vaporized inaccordance with the present invention, so that the cathode metalreturned to the electrolytic cell has the same composition as thecathode metal utilized under normal operating conditions at the inlet.In this way, by eliminating all of the product of electrolysis,

that is, that additional amount of sodium metal present in the cathodemetal from the electrolysis of the fused sodium chloride, it is possibleto have a continuously running operation without any appreciablevariation of the electrolyte metal in the operating cathode. This, ofcourse, is a desirable feature of the process of the present inventionwhich allows the electrolytic cell to be run continuously with theemployment of a cathode metal of a constant composition.

The amount of sodium metal evaporated from the alloy of cathode metaland electrolyte metal can be easily controlled through a simple controlof the degree of vacuum employed in the flash vaporizer in accordancewith the process of the present invention, as well as the temperature ofthe alloy entering the flash vaporizer from the electrolytic cell. Forthis purpose, an optional heating element can be employed to heat thealloy leaving the electrolytic cell to that temperature desired for thenecessary flash vaporization.

The present invention has the further feature that it serves as aninherent stable control on the operation of the process, assuring thatit will be smooth without requiring manual control or mechanical controlinstruments of usual types. For instance, if the electrolytic currentshould be increased, generating sodium at a greater rate thanpreviously, the concentration of sodium in the lead entering thevaporization chamber would rise, increasing its vapor pressure, thusincreasing the rate of vaporization, thus increasing the density rise ofthe alloy in the vaporization chamber, thus increasing the rate ofnatural circulation, thus after a moderate interval reestablishingsodium concentrations in the alloy streams entering and leaving thevaporization chamber and thus the electrolytic cell, close to theconcentrations in the alloy streams before the rise in current. Thefinal result of the rise in current thus will be a substantiallyproportional increase in alloy flow rate, to yield substantially thesame sodium concentrations in and out of the cell as before, whichpresumably are desired operating concentrations. Such an increase inalloy flow rate would not occur automatically with any pumping system,but would require a control instrument sensitive to sodium concentrationcontrolling the pump or a valve in the piping.

The process of the present invention will now be described withreference to the drawings wherein:

FIG. 1 is a diagrammatic representation of the process of the presentinvention; and

FIG. 2 is an enlarged view of a modified flash vaporizer or stillemployed in accordance with the process of the present invention.

In the figures, like numerals represent like elements.

The process of the present invention is diagrammatically illustrated inFIG. 1. In FIG. 1, the electrolytic cell is simply represented as 1,such cell containing a flowing cathode 5 on which rests the fusedelectrolyte salt 3. For purposes of the discussion of the drawings, itwill be assumed that the cathode metal is lead and the electrolyte saltis sodium chloride. For simplicity in this drawing, anodes spaced abovethe flowing molten electrolyte, and the inlet or outlet of the cell fromwhich the initial cathode metal and electrolyte as well as subsequentelectrolyte are sent into the electrolytic cell and where the chlorinegas evolved is given off and collected are not shown.

As shown in FIG. 1, the molten alloy of cathode metal and theelectrolyte metal, that is, of lead and sodium, is withdrawn from theelectrolytic cell 1 through line 7 by the vacuum pump operating on thesystem, the vacuum pump not being illustrated in FIG. 1. The alloy thenenters an evaporator or flash still 9 operated without the addition ofheat and at an absolute pressure of about 40 to about lOO torr (mm. Hg),such reduced pressure being obtained by the vacuum pump. Due to the hightemperature of the alloy and the vacuum conditions within the evaporatoror flash still 9, a portion of the sodium within the alloy will flashoff and exit from the evaporator or flash still 9 through line 13.

Due to the flashing off of the sodium vapor, the liquid alloy 11 withinthe evaporator or flash still 9 will cool, and become more dense alsodue to the lower concentrations of the lighter sodium metal. The moredense alloy, corresponding substantially to the composition of theliquid cathode employed during the normal operation of the electrolyticcell, continually passes back to the electrolytic cell throughgravitational forces alone through line 21, which may be mostly withinthe cell. This withdrawal of the alloy rich in sodium and return of thecathode metal having a composition for use in normal operation of thecell, without the need of special pumps or special equipment to effectthe same, comprises the essential features of the natural circulationsystem of the present invention.

The sodium vapor passing out of the evaporator or flash still 9 throughline 13 passes under condenser 15 and subsequently receiving vessel 17,both of which are maintained under vacuum conditions. Thus, the vacuumpump not illustrated in Figure 1 pulls a vacuum through line 19, thereceiver 17, condenser 15, vapor line 13 and evaporator or flash still9, so that the entire system is maintained under vacuum conditions. Aspointed out previously, the sodium metal produced in accordance withthis process and collected in receiver 17 is of a purity which can bedirectly used in many industrial processes, wherein exceptionallyhigh-purity sodium is not required. Thus, the sodium metal producedhaving from about 0.1 to about 1 percent by weight lead can beconveniently employed, for example, in the industrial process utilizedin the manufacturing of valuable tetraethyl lead. Furthermore, if higherpurity is required, an additional vaporization stage can be added, asdescribed in copending application, Ser. No. 586,859, now US. Pat. No.3,484,233, or the sodium produced can be subsequently purified by anindependent distillation operation, as previously pointed out.

As also pointed out previously, it is sometimes necessary to heat themolten alloy exiting from the electrolytic cell so that the same, whenintroduced into the evaporator or flash still, will be at a temperatureappropriate for the flash vaporization of the required amount of sodium.For this purpose line 7 may be equipped with a heater 25 which caneffect the heating of the alloy composition. This heat will hardlyaffect the natural circulation forces. Similarly, due to the cooling ofthe cathode metal upon the evaporation of the sodium it may be desirableto heat the returning cathode metal prior to introduction into theelectrolytic cell. For this purpose, therefore, line 21 may beoptionally equipped with a small heating element 27 to heat the cathodemetal returning to the electrolytic cell to the proper temperature.Since, however, the amount of cathode metal returning to theelectrolytic cell will be quite small in comparison to the body ofliquid cathode metal present in the cell, it generally is not necessaryto effect the heating of the recirculating cathode metal to stabilizecell temperatures.

- Furthermore, heating of return line 21 slightly diminishes the naturalcirculation forces, so would need to be minimized.

Convenient, rapid and reliable control of the level of the cathode metalin the electrolytic cell is important in the operation of this process.It is possible to alter the level in the electrolytic cell by mechanismswithin the cell, or by receivers connected to pipes 7 or 21, etc.However, it would be more desirable to operate on the cathode metalalloy in the vaporizer itself by varying its volume while maintainingits composition constant, or by changing its composition while keepingits volume in the vaporizer constant, or by simultaneous changes in bothcomposition and vaporizer volume.

Variations in the level in the electrolytic cell can be carried out withthe arrangement of FIG. 1 by changing the flow of air to the sodiumcondenser, which changes the vacuum in the vapor space in the samedirection. An increase in condenser airflow, which increases the vacuum(or decreases the absolute pressure) will cause the level in thevaporizer to rise (and simultaneously will gradually cause the cathodemetal composition to decrease in sodium content to a new lowerequilibrium composition). There may thus be an immediate, and possiblyundesirably rapid, change in cathode metal level, unsuitable for acontinuing smooth control of the process.

In FIG. 2 a modification of the vacuum still or vaporizer is shown whichpermits a somewhat simpler and more gradual control of the level of thecathode metal in the electrolytic cell. In this design the level of thepool will remain substantially constant at the top of the outlet pipe21. Accordingly, when the condenser air flow is adjusted in the desireddirection a gradual change in the sodium content of the cathode metalwill begin to occur, and the level of the cathode metal in theelectrolytic cell will begin to change and can be permitted to continueas long as desired. The level of the cathode metal in the exit pipe 21is shown at 33 and will vary somewhat in operation. However, the volumeof the pipe is small, so such variation will have little effect on thelevel of cathode metal in the electrolytic cell.

Circular fixed bafile 29 surrounding pipe 21 is a desirable additionalfeature. Baffle 29 rises above the highest possible level in chamber 9,and descends possibly halfGway to the floor of the chamber 9. Bafflc 29prevents the floating matter or lighter alloy entering through pipe 21from floating and channeling over to pipe 21 and running quickly downpipe 21 without releasing sodium to the same extent as the rest of thestream. Bafile 29 assures that if there is stratification of the alloyin vaporizer 9 only dense alloy, collecting just below baffle 29, willflow down pipe 21. The rest of the volume below baffle 29 is availablefor any accumulation of dense dross, which can be removed therefromduring any needed periodic cleaning of the system. As another possiblefeature, a flash vaporization surface 31 is added around the top of pipe7, as shown in FIG. 2.

It may be noted that a variation in the constant cathode metal volumeretained in the vaporizer as per the design of FIG. 2 can beincorporated by employing an externally adjustable or changeable lengthfor pipe 21, as by a fairly closefitting sleeve which can be raised orlowered in or around the top of pipe 21 by a handle extending through astufiing box in the vaporizer roof, not shown in FIG. 2 but readilyvisualized. Alternatively, pipe 21 can be manifold rising alongsidevessel 9 and connected to it through several horizontal valves atdifferent levels, the valve at the best level being open at any onetime.

The process of the present invention will now be described by referenceto the following specific example.

Using a system such as illustrated in FIG. 1, an electrolytic cell wasoperated by the introduction initially of a cathode compositioncomprising an alloy of 90 percent by weight molten lead and percent byweight sodium. The electrolyte employed in a horizontal electrolyticcell was molten sodium chloride which was continuously introduced intothe cell as a layer flowing on the molten cathode composition betweenthe same and the anodes.

By pulling a vacuum through the receiver, sodium vapor condenser, andvaporizer and applying cooling air to the sodium condenser so as tomaintain a pressure in the vaporizer of 70 mm. Hg., the cathodecomposition rich in the electrolyte metal, i.e. a composition containingabout l0. 1 3 percent sodium, was withdrawn into the vaporizer in whichflash vaporization of the sodium metal takes place. Due to the increasein density of the cathode composition based upon the vaporization of thelighter sodium metal and the decrease in temperature due to the flashvaporization, a cathode composition approximating that initiallyintroduced into the electrolytic cell, i.e. 90 percent by weight leadand 10 percent by weight sodium, was recirculated from the vaporizer.The sodium vapor withdrawn from the flash vaporizer was condensed andcollected in a receiver to yield a sodium product containingapproximately 0. l-l percent by weight lead.

By this process, it is possible to continuously operate the electrolyticcell through the continuous recirculation of a cathode composition, thesame as initially introduced into the cell. Accordingly, by the naturalrecirculation of the cathode composition based upon increase in densityassociated with a cooling of the same and a vaporization of the lighterelectrolyte metal, it is possible to operate the electrolytic cell in asimple and efficient manner.

While the present invention has been described primarily with respect toa system wherein the cathode metal comprises lead or a mixture of leadand sodium and the electrolyte comprises sodium chloride, it is ofcourse possible to utilize other cathode metals and electrolytes inaccordance with the process of the present invention. Thus, for example,the electrolyte metal employed in accordance with the process of thepresent invention can conveniently be other alkali metals, e.g. lithiumor potassium or alkaline earth metals such as calcium; in addition,mixtures can be employed such as a mixture of sodium and potassiumchlorides, in which case the sodiumpotassium alloy, known as NaK, can bedirectly produced. Furthermore, the heavy metal molten cathode caninclude other metals such as zinc or tin instead of the conventionallyemployed lead.

As indicated previously, the process of the present inven tion uniquelyprovides for a simple and efficient natural recirculation system forcontinuous operation of an electrolytic cell employing a flowing moltenmetal cathode. Particularly, when sodium is employed as the electrolytemetal, as in the electrolysis of sodium chloride, the process of thepresent invention provides a simple and economical manner for theproduction of a grade of sodium, which is entirely satisfactory withoutfurther purification for a number of industrial applications. Thus, thesodium produced in accordance with the process of the present inventioncontaining from about 0.1 percent to about 1 percent by weight lead canbe conveniently employed without further purification directly in theproduction of tetraethyl lead.

We claim:

1. A process for the purification and recirculation of the cathode metalof an electrolytic cell utilizing a flowing molten metal cathode, whichprocess comprises withdrawing under vacuum conditions a portion of thecathode metal containing, as a product of the electrolysis reaction,electrolyte metal; vaporizing the electrolyte metal under vacuumconditions and without the addition of heat so as to remove all of theproducts of electrolysis and provide a cathode metal of a compositionfor recirculation to the electrolytic cell; and recirculating thecathode metal by gravitational forces through the increase in densityassociated with the vaporizing of the electrolyte metal and decrease inthe temperature of said recirculated cathode metal.

2. The process of claim 1 wherein said cathode metal comprises lead andsaid electrolyte metal, sodium.

3. The process of claim 2 wherein the condensation of the vaporizedelectrolyte metal produces a sodium product containing from 0.1 to 1percent by weight lead.

4. A process of operating an electrolytic cell employing a flowingmolten metal cathode and a molten electrolyte salt, which processcomprises introducing into the electrolytic cell a cathode compositioncomprising a molten cathode metal and the metal of the electrolyte salt;withdrawing under vacuum conditions a portion of the cathode compositionricher in the electrolyte metal as a result of the electrolysis of theelectrolyte salt; vaporizing a portion of the electrolyte metal from thewithdrawn cathode composition under vacuum conditions and without addedheat so as to remove all of the electrolyte metal resulting from theelectrolysis of the electrolyte salt and to provide a cathodecomposition for recirculation to the electrolytic cell of the samecomposition as that initially introduced; and recirculating the cathodemetal composition by gravitational forces through an increase in densityassociated with the vaporizing to the electrolyte metal and decrease intemperature of said recirculated cathode metal.

5. The process of claim 4 wherein said cathode composition compriseslead containing a minor amount of sodium and said electrolyte metalcomprises sodium.

6. The process of claim 5 wherein said cathode composition comprisesmolten lead containing approximately 10 percent by weight sodium.

7. The process of claim 5 wherein the condensation of the vaporizedelectrolyte metal produces a sodium product containing from 0.1 to 1percent by weight lead.

2. The process of claim 1 wherein said cathode metal comprises lead andsaid electrolyte metal, sodium.
 3. The process of claim 2 wherein thecondensation of the vaporized electrolyte metal produces a sodiumproduct containing from 0.1 to 1 percent by weight lead.
 4. A process ofoperating an electrolytic cell employing a flowing molten metal cathodeand a molten electrolyte salt, which process comprises introducing intothe electrolytic cell a cathode composition comprising a molten cathodemetal and the metal of the electrolyte salt; withdrawing under vacuumconditions a portion of the cathode composition richer in theelectrolyte metal as a result of the electrolysis of the electrolytesalt; vaporizing a portion of the electrolyte metal from the withdrawncathode composition under vacuum conditions and without added heat so asto remove all of the electrolyte metal resulting from the electrolysisof the electrolyte salt and to provide a cathode composition forrecirculation to the electrolytic cell of the same composition as thatinitially introduced; and recirculating the cathode metal composition bygravitational forces through an increase in density associated with thevaporizing to the electrolyte metal and decrease in temperature of saidrecirculated cathode metal.
 5. The process of claim 4 wherein saidcathode composition comprises lead containing a minor amount of sodiumand said electrolyte metal comprises sodium.
 6. The process of claim 5wherein said cathode composition comprises molten lead containingapproximately 10 perceNt by weight sodium.
 7. The process of claim 5wherein the condensation of the vaporized electrolyte metal produces asodium product containing from 0.1 to 1 percent by weight lead.